1. Field of the Invention
This invention relates to a motor generating mechanical power without resorting to electromagnetic power, and more particularly to a bar-shaped ultrasonic motor (vibration wave driven motor) which utilizes circular motion excited in a vibrator by the combination of expansive and contractive vibrations in the axial direction to rotate a driven member fitted coaxially with the vibrator by frictional driving.
2. Related Background Art
A motor as shown for example in FIG. 13 of the accompanying drawings has heretofore been proposed as an ultrasonic motor (vibration type motor) of this type in U.S. patent application Ser. No. 586,303.
In FIG. 13, the reference numeral 1 designates a vibration member comprising a metallic round bar having a small-diametered shaft portion 1a forming a fore end portion, a large diametered shaft portion 1b forming a rear end portion, and a horn-shaped horn portion 1c formed between the small-diametered shaft portion 1a and the large-diametered shaft portion 1b and having a diameter progressively decreasing toward the fore end portion, the reference numeral 2 denotes a keep member comprising a metallic round bar formed to the same outer diameter as the large-diametered shaft portion 1b of the vibration member 1 and having a bolt insertion hole 2a formed along the axis thereof, the reference numerals 3 and 4 designate circular ring-shaped piezo-electric element plates formed to the same outer diameter as the large-diametered shaft portion 1b, and the reference numeral 5 denotes the electrode plate of the piezo-electric element plates 3 and 4. The piezo-electric element plates 3 and 4 with the electrode plate 5 interposed therebetween are disposed between the vibration member 1 and the keep member 2, and the keep member 2 is fixed to the vibration member 1 by a bolt 6, whereby the piezo-electric element plates 3 and 4 are fixed between the vibration member 1 and the keep member 2 to thereby constitute a vibrator A. The bolt 6 has its head in contact with the keep member 2 with a circular ring-shaped insulator 7 interposed therebetween and has its shank portion held in non-contact with respect to the piezo-electric element plates 3, 4 and the electrode plate 5.
The piezo-electric element plates 3 and 4 each have on one surface thereof two electrodes (plus electrode a and minus electrode b) differing in the direction of polarization from each other and polarized in the direction of thickness, said two electrodes being symmetrically formed on the opposite sides of an insulating portion d formed on the center line, and have formed on the other surface thereof an electrode c common to the plus electrode a and the minus electrode b, and are disposed with a positional phase difference of 90.degree. therebetween with respect to the axis of the vibrator A. The polarized electrodes (the plus electrode a and the minus electrode b) of the piezo-electric element plate 3 are in contact with the rear end surface of the vibration member 1 which is an electrical conductor, and the piezoelectric element plate 4 is in contact with the front end surface of the keep member 2 which is an electrical conductor.
An AC voltage V.sub.1 is applied to between the electrode plate 5 and the vibration member 1 and an AC voltage V.sub.2 is applied to between the electrode plate 5 and the keep member 2, whereby the vibrator A is vibrated by the combination of vibration caused by the expansive and contractive displacement of the piezo-electric element plate 3 in the direction of thickness thereof and vibration caused by the expansive and contractive displacement of the piezo-electric element plate 4 in the direction of thickness thereof.
The AC voltage V.sub.1 and the AC voltage V.sub.2, as shown in FIG. 14 of the accompanying drawings, are identical in amplitude and frequency and have a difference of 90.degree. in time and spatial phases therebetween.
Thus, the vibrator A makes circular motion like that of the rope used in rope skipping (hereinafter referred to as the rope-skipping) about the axis thereof. The principle on which such circular motion occurs is described in detail in the above mentioned U.S. application Ser. No. 586,303, etc. and therefore need not be described herein.
As shown in FIG. 15 of the accompanying drawings, a rotor 8 is fitted coaxially with the axis l of the vibrator A, and the rear end portion (hereinafter referred to as the frictional contact portion) 8b of the inner diameter portion of the rotor 8 extends to a location corresponding to a sliding portion B, and the frictional contact portion 8b is brought into contact with the sliding portion B of the horn portion 1c. The horn portion is provided to obtain an appropriate frictional force in the sliding portion B by being subjected to an axial pressure force. This sliding portion B provides the loop of the rope-skipping in the vibration member 1.
The bore of the inner diameter portion 8a of the rotor 8 is of such structure that in the vibration member 1, it contacts with the position of the mode of the rope-skipping with a member 8d of low coefficient of friction interposed therebetween, and the rotor 8 is provided with an escape 8c to prevent the inner diameter portion from contacting with any vibration created in the other portions than the sliding portion B and producing of sounds.
The frictional contact portion 8b of the rotor 8 diverges into such a shape that the inner diameter thereof conforms to the outer peripheral shape of the sliding portion B, which progressively increases, and surface-contacts with the sliding portion B during the rope skipping motion of the vibration member 1.
The rotor 8 is pushed for example, in the direction of arrow in FIG. 15 by a spring or the like, not shown, through a thrust bearing, not shown, thereby producing a predetermined frictional force in the portion of contact between the frictional contact portion 8b and the sliding portion B by the sliding portion having the aforedescribed appropriate progressively increasing diameter, and also is permitted axially rotate by the thrust bearing.
From the above-described structure, there is realized an ultrasonic motor (a vibration wave driven motor) in which the vibration of the vibration member 1 is transmitted as a rotational force to the frictional contact portion 8b of the rotor to thereby rotate the rotor.
Generally, however, the ultrasonic motor (vibration type motor) of this kind has a resonance frequency of the order of several tens of kilohertz, and unless it is driven in the vicinity of this frequency, a great amplitude will not be obtained and such motor will not operate as a motor. Also, the resonance frequency of the motor fluctuates depending on environmental conditions such as temperature and humidity and load conditions.
This leads to the problem that the number of rotations become unstable if the motor is driven at a predetermined frequency.